1. Field of the Invention
The present invention relates to a shock absorber capable of producing a variable damping force, for use between a vehicle body and a road wheel of a motor vehicle, and more particularly a hydraulically operated displacement transmission mechanism in such a shock absorber which has a means for automatically compensating for a leakage of working oil from a hydraulic pressure chamber which is compressible by a piezoelectric actuator in the form of a stack of piezoelectric elements.
2. Prior Art
As is well known in the art, some automobiles are equipped with adjustable shock absorbers which can vary or adjust damping forces depending on vibrating or running conditions of the automobiles to give better riding comfort to automobile drivers and passengers. Such adjustable shock absorbers are disclosed in Japanese Laid-Open Patent Publications Nos. 60(1985)-143237, 60(1985)-143238, and Japanese Laid-Open Utility Model Publication No. 61(1986) -67009, for example. Some adjustable shock absorbers incorporate a piezoelectric actuator composed of a stack of piezoelectric elements (also referred to as a "piezoelectric stack") for actuating a damping force varying valve for an increased response to an input signal, as disclosed in Japanese Laid-Open Patent Publication No. 61(1986)-85210 and U.S. Pat. No. 4,729,459, for example.
The piezoelectric stack itself does not produce a sufficient displacement when a voltage is applied thereto, and hence is not used directly as a damping force varying valve. Instead, the displacement of the piezoelectric stack is amplified by working oil and applied to a plunger.
More specifically, the piezoelectric actuator comprises a housing defining a cylindrical storage chamber filled with working oil, and a piezoelectric stack disposed in the storage chamber and having one end fixed to the housing. A piston is fitted over the opposite free end of the piezoelectric stack and slidably disposed in the storage chamber. The piston and the housing define therebetween a hydraulic pressure chamber into which one end of a plunger faces. The plunger is axially slidably fitted in a sleeve. When a voltage is applied to the piezoelectric stack, the piston is moved to cause working oil in the hydraulic pressure chamber to move the plunger axially for thereby varying damping forces of the shock absorber. The piston and the plunger can be returned to their original positions by respective springs.
Since the diameter of the plunger is much smaller than the diameter of the piston, the displacement of the piezoelectric stack is amplified by the working oil in the hydraulic pressure chamber and applied to the plunger.
However, it has been difficult to seal the hydraulic pressure chamber completely. Even if an O-ring is placed around the plunger, the working oil in the hydraulic pressure chamber gradually leaks out along the peripheral surface of the plunger during repeated axial movement of the plunger. Therefore, the hydraulic pressure chamber becomes negative in pressure, and the plunger is displaced gradually decreasing strokes while the voltage applied to the piezoelectric stack has constant peak values. When this happens, the damping forces cannot sufficiently be varied or switched over.
Even if the hydraulic pressure chamber is completely sealed off against working oil leakage, another problem occurs. When the ambient temperature rises, the working oil in the hydraulic pressure chamber expands and the plunger is undesirably displaced by the expanding working oil. Therefore, the damping forces of the shock absorber may be varied when they should not be varied.